1. Field of the Invention
This invention relates to austenitic iron-base cryogenic alloys. More particularly, it relates to such alloys which show superior Charpy V-notch impact strength at -320.degree. F. My alloy is particularly useful as a weld deposit, in which form it is compatible in cryogenic properties with austenitic stainless steel and ASTM A203E steel base plates. The invention also relates to a tubular composite arc welding electrode which deposits such alloy when used in an arc welding process.
2. Description of the Prior Art
In weld fabrication of magnet cases for fusion reactors, such as those of the Tokomak design, which in use are cooled by liquid helium (-452.degree. F.), it is necessary to use weld filler metal which is compatible with the plate material in both tensile strength and impact toughness. Typical of plate material used in such applications are austenitic stainless steels such as AISI 304L and 316L, both of which meet 70,000 psi minimum tensile strength. In addition to meeting such tensile requirements, weld metal used for joining these steels must exhibit at least 15 mils lateral expansion when tested in the Charpy V-notch impact test at -320.degree. F. Details of the Charpy test, which is well known and commonly used in the metallurgical field, are set forth in ASTM E23-72.
In addition to the fusion reactor applications, there are increasing numbers of cryogenic applications requiring iron-base alloys, either as plate material or as weld filler metal. Satisfactory plate material is often available for such applications, for example the low carbon austenitic stainless steels mentioned above and their regular carbon equivalents, and ASTM A203E steel which contains about 31/2% nickel. Available weld filler metals, however, have not heretofore been completely satisfactory for a number of reasons. Prior workers have sometimes used nickel base alloys such as INCONEL* 82 (AWS ENiCr-3) as filler metals; although such filler metals have satisfactory cryogenic properties, they are extremely expensive, particularly when used with relatively low cost plate material such as AISI 304L and 316L. Filler metals matching the chemistry compositions of the plate material such as the austenitic stainless steel grades are generally deficient in impact toughness, due to the metallurgical structures which they develop in weld deposition. FNT *Registered Trademark of the International Nickel Company.
A few specialized austenitic iron-base filler metal alloys have been developed for use in covered electrode weld joining of cryogenic plate of the above discussed types. Typical of such alloys is one which meets the American Welding Society (AWS) chemical limits for type 316L covered electrode weld metal (AWS A5.4) but is specially balanced to have manganese and nickel at the high side of the acceptable ranges and chromium and nitrogen at the low side of such ranges. This alloy, while generally satisfactory when deposited by covered electrodes, has not been successful when deposited using tubular composite electrodes in automatic and semiautomatic arc welding processes; the reasons for such lack of success are not completely understood, but it is known that the alloy is sensitive to nitrogen, which ruins its impact strength, and the automatic and semiautomatic arc welding processes often produce deposits with higher nitrogen levels than comparable covered electrode deposits.
From the foregoing it can be seen that a need exists for an iron-base alloy which combines reasonable cost with acceptable tensile strength and cryogenic impact properties when weld deposited using tubular composite electrodes. Such alloy must be relatively insensitive to nitrogen pickup and, because several of the cryogenic plate materials such as ASTM A203E are often stress relieved after fabrication, should retain its properties after stress relief treatment; the latter requirement dictates that an acceptable alloy contain very little or no delta ferrite in the as-weld deposited condition, since such ferrite transforms to the brittle sigma phase during stress relief and thereby severely lowers impact strength of the weld joint.